Crystalline silicon-based solar cells using crystalline silicon substrates are high in photoelectric conversion efficiency, and thus have been widely and generally used in solar power generation systems. Among them, a crystalline silicon-based solar cell in which conductive amorphous silicon-based layers having a band gap different from that of a single-crystal silicon are formed on surfaces of the crystal silicon substrate is called a heterojunction solar cell.
Among heterojunction solar cells, a solar cell having an intrinsic amorphous silicon layer between a conductive amorphous silicon-based layer and a crystal silicon substrate is known as one embodiment of a crystalline silicon-based solar cell with a high conversion efficiency. By forming an intrinsic amorphous silicon layer between a single-crystal silicon substrate and a conductive amorphous silicon-based layer, generation of defect levels during formation of the conductive amorphous silicon-based layer can be reduced and defects present on the surface of single-crystal silicon (principally dangling bonds of silicon) can be terminated (passivated) with hydrogen. In addition, due to a presence of an intrinsic amorphous silicon layer, carrier-introduction impurities can be prevented from diffusing to the surface of single-crystal silicon at the time of forming a conductive amorphous silicon-based layer.
In the aforementioned heterojunction solar cell, a transparent electroconductive layer is formed on the surface of a conductive amorphous silicon-based layer. The transparent electroconductive layer preferably has a high optical transparency and a low resistance. As a material thereof, a transparent conductive metal oxide such as a crystalline indium tin complex oxide (ITO) is used. Generally, a collecting electrode is formed on the transparent electroconductive layer. For the collecting electrode, an Ag paste or the like is used as a material.
As a technique related to a manufacturing method of a solar cell, it is known that a heat treatment is carried out in an air atmosphere after formation of an electrode. For example, Patent Document 1 discloses that in manufacture of a solar cell using a crystalline silicon substrate, a heat treatment is carried out at 300° C. or higher and 700° C. or lower under a hydrogen atmosphere after an electroconductive paste is applied. It is reported in Patent Document 1 that since a metal oxide contained in a glass frit in the electroconductive paste is subjected to hydrogen reduction by the heat treatment, both of high bonding strength and low electrical contact resistance are achieved. With respect to a manufacture of a flexible solar cell, Patent Document 2 discloses that a transparent electrode is made less resistant by carrying out a heat treatment after formation of the electrode. Patent Document 3 discloses that when a thin-film solar cell using microcrystalline silicon as a photoelectric conversion layer and a conductivity-type layer is subjected to a heat treatment under a low-oxygen partial pressure atmosphere for one hour or more, dope impurities in the conductive microcrystalline silicon layer are activated to improve electrical characteristics.